1. Field of the Invention
This invention relates to apparatus for operating a gas discharge lamp, such as a fluorescent light, a mercury vapor lamp, a sodium lamp, or a metal halide lamp.
2. Description of the Prior Art
Control circuits for gas discharge lamps are known which obviate the need for the usual heavy and expensive series ballast devices, corresponding to the inductor in this device. In such circuits, switching elements are provided to periodically reverse the direction of current through the lamp to reduce the deterioration or errosion of electrodes, and to ensure a high enough frequency of switching to reduce the requirement for the size of the ballast. Such circuits generally require two switching elements for each direction of the current.
Attempts have been made to fabricate the same type of circuit using only a single switching element to cause current reversal in the lamp. For example, the patent to D. B. Wijsboom, U.S. Pat. No. 3,906,302, is directed to such an arrangement and incorporates an inductor in parallel with the lamp, which lamp is in series with a switching device. Such a switching device is generally operated at relatively high frequencies, such as 20 kHz.
One problem has been that the fly back voltage during current reversal required to ignite the lamp when the circuit is first activated must be large enough to generate a sufficiently strong voltage gradient in the lamp to ionize the gas. This causes a large voltage to appear across the switching device which can damage the device during ignition, thereby limiting the reliability of the control circuit.
Another problem has been that it is often necessary to reduce the voltage supplied to the circuit in order to ensure that only the optimum lamp voltage is supplied to the lamp. It has been found that such a reduction in supply voltage decreases the voltage gradient in the lamp for starting ignition of the lamp during current reversal. Therefore, with the introduction of a step down auto transformer, the fly back voltage of the circuit must be increased to provide a sufficient voltage gradient in the lamp. Such an increase in fly back voltage increases the wear in components in the circuit and a consequent loss of reliability.